A healthy cardiac cycle includes a period of passive filling of the heart during atrial and ventricular diastole, followed by atrial contraction, which forces blood through the atrioventricular valves into the ventricles. Following atrial contraction, the ventricles contract, expelling blood from the heart into the rest of the body.
Some patients have implantable pacemakers that deliver electrical stimuli or paces to the atria or the ventricles or both, causing the chambers to activate. In general, a pacemaker delivers paces to promote atrioventricular synchrony, i.e., to cause the atria and ventricles to contract in a cooperative and efficient fashion. In some circumstances, however, pacing fails to cause atrioventricular synchrony, and instead causes the atria and the ventricles to activate out of sequence at approximately the same time. The result is a phenomenon known as “pacemaker syndrome.”
Activation of the atria and ventricles at substantially the same time causes the atria and ventricles to work against one another. Instead of ejecting the blood into the body, the ventricles force some blood backward through the atrioventricular valves and into the atria. As a result, a patient suffering from pacemaker syndrome generally experiences adverse effects due to reduced hemodynamic function of the heart, such as weakness, dizziness and shortness of breath.